(1) Field of the Invention
The present invention relates to a Si--SiC material of Si concentration-gradient type and a SiC fiber-reinforced Si--SiC composite material of Si concentration-gradient type, both superior in properties such as weatherability, oxidation resistance, creeping resistance, strength, toughness and the like, as well as to processes for production of said materials.
(2) Description of Related Art
In the midst of rapid progress of technological innovation, big projects are being planned and carried out in various parts of the world for development of state-of-the-art technologies such as space shuttle and space plane (in the field of space development), high-temperature combustion gas turbine (in the field of energy) and high-temperature gas furnace and nuclear fusion reactor (in the field of atomic energy).
Also, utilization of hydrogen energy is being studied in order to use it as an energy other than nuclear energy and solar energy. In this connection, expensive metal or fine ceramic is being investigated to use it as a reactor material. These structural materials must have high strength at intermediate to high temperatures (200 to 2,000.degree. C.), high reliabilities in toughness and impact resistance, and resistances to environment (e.g. corrosion resistance, oxidation resistance and radiation resistance).
Currently, as a ceramic superior in heat resistance, there are newly developed ceramics, i.e. silicon nitride and silicon carbide both having high strength. They, however, are inherently fragile and are very fragile even when there have small flaws, and further have low resistance to thermal or mechanical impact.
In order to overcome these drawbacks of the above ceramics, there was developed a ceramic matrix material (CMC) wherein a continuous ceramic fiber is mixed with a ceramic. This material, having high strength and high toughness even at high temperatures, excellent impact resistance and excellent resistances to environment, is under active study as a structural material having an ultrahigh resistance to heat, in Europe, U.S.A., etc.
For example, there was developed a ceramic matrix composite (CMC) wherein a fiber was mixed into a ceramic matrix, by making ceramic long fibers (ordinarily several hundreds to several thousands fibers) having a diameter of about 10 .mu.m, into a fiber bundle (a yarn), arranging these fiber bundles two-dimensionally or three-dimensionally to form a unidirectional sheet or a cloth, or laminating a plurality of such sheets or such cloths to form a preliminary molded material (a fiber preform) having a desired shape, and forming, inside the preliminary molded material, a matrix by, for example, (a) chemical vapor infiltration or (b) inorganic polymer infiltration and sintering, or filling the inside of the preliminary molded material with a ceramic powder by casting and sintering the resulting material to form a matrix inside the preliminary molded material.
In the sintering (firing) used in production of conventional ceramic matrix material, however, no attention was paid to the CO gas generated in the process, and it was conducted only to introduce an inert gas by a slight pressure control mainly for prevention of Si vaporization.
Consequently, the CO gas produced during firing in association with the conversion of organic polymer into ceramic, by the reaction of free carbon (present in firing atmosphere) and O.sub.2 and the reaction of free carbon and SiO.sub.2, is liberated to form defects; these defects and the growth of .beta.-SiC crystals bring about significant deterioration of strength; further, the pores in produced ceramic matrix material cannot be reduced to zero and their size is as large as about 1 mm, inviting deterioration of weatherability and oxidation resistance.
Further, although the conventional ceramic matrix material contains a SiC fiber inside, the thermal stress which the material receives during actual use, is caused by the difference in thermal expansion between SiC fiber and Si--SiC moiety; therefore, the material has had laminar peeling in some cases.